A battery separator functions to prevent contact between the anode and the cathode, yet allows the transfer of certain ions that are needed to maintain consistent battery discharge. While the role performed by the battery separator is essentially passive, the design of the separator is important to provide batteries with long and consistent performance, and to substantially eliminate shorts caused by a breach in the separator. Among other requirements, the separator should be able to withstand the harsh environment of the battery, be resistant to attacks by oxidizing agents, allow the transmission of necessary ions, and have a low electrical resistance. To these ends, various separators for batteries have been described in the art.
Historically, regenerated cellulose separators were used because of their low electrical resistance and ability to conduct hydroxyl ions in alkaline environments. However, regenerated cellulose has a very low permeability to hydrogen, which is prohibitive for certain battery designs.
Following is a brief summary of references relevant to the field of battery separators.
U.S. Pat. No. 7,070,884 to Thompson, et al. describes a separator which comprises a nonwoven fabric composition having a layer of fine spunbond filaments, and a layer of thermoplastic meltblown microfibers.
Polymeric battery separator films are also well known in the art. For example, U.S. Pat. No. 4,505,998 to Hsu et al. is directed to a battery separator which is made from films cast from a cross-linked polymer having vinyl alcohol units and carboxylic acid units.
A similar approach is described in U.S. Pat. No. 5,290,645 to Tanaka et al., which relates to a battery separator that includes a hydrophilic portion and a hydrophobic portion. The hydrophilic portion is made from cross-linked polyvinyl alcohol copolymer, where the copolymer contains polymerizable cationic crosslinking compounds. The Tanaka et al. '645 patent states that the crosslinking prevents the erosion of the film by the alkaline electrolyte. U.S. Pat. No. 6,033,806 to Sugiura et al. likewise discloses a separator for batteries comprising a film of cross-linked polyvinyl alcohol having a degree of saponification of about 70-98.5%, where the polyvinyl alcohol is contacted with an oxidizing agent to cleave 1,2-diol units. The separator is reportedly durable in an alkaline environment.
U.S. Pat. No. 6,607,859 to Tanaka et al. describes a separator for a battery which is made from a fabric that includes polyolefin fibers which are treated to render them hydrophilic. As stated in '859 Tanaka et al., the hydrophilic treatment may include sulfonation, fluorination, treatment with a surfactant, or corona discharging treatment. For example, the sulfonating treatment may entail contacting the fibers with fuming sulfuric acid, and the fluorination may be performed by contacting the fibers with diluted fluorine gas. The hydrophilic treatment is likely necessary to enable sufficient wetting of the electrolyte on the hydrophobic polyolefin fibers.
U.S. Pat. No. 7,029,792 to Cheiky et al. discloses a membrane separator for a zinc-anode containing battery, where the separator is made from a mixture of (1) cellulose, and (2) a hydrophilic polymer, such as ethyl cellulose. According to the Cheiky et al., '792 patent, the separator exhibits good hydroxyl conductivity and hydrogen transport.
U.S. Pat. No. 7,052,800 to Harada et al. discloses a separator for a battery with a nickel cathode. The separator includes a laminate of a sulfonated substrate and a porous hydrophilic film. The sulfonated substrate is generally a polyolefin that is sulfonated with sulfuric acid. The hydrophilic film includes resins such as polyamide. The Harada et al. '800 patent states that the sulfo group traps nitrogen ion-containing impurities which are present in nickel cathode batteries.
Other references of interest may include U.S. Pat. No. 6,523,699 to Akita et al., U.S. Pat. No. 6,878,483 to Morokuma et al., and United States Patent Application Publication No. 2006/0147804 to Yamamoto et al.
Notwithstanding the above-noted advancements in this field, a need remains for separators that are suitable for use in batteries, which exhibit improved resistance to oxidation, and which improve the conductivity of the battery. Batteries usually have strong oxidation causing components that can degrade the separator and cause the battery to short. Moreover, the degradation process is enhanced at elevated temperatures, as are common during battery discharge. Commercially available batteries typically include numerous separator components to act as back-ups when one of the separators fails, i.e., the separators are sacrificial. This practice takes up valuable space in the battery which could otherwise be used to enlarge the anode and/or the cathode.
According to the invention, a battery separator is provided with a film or coating that comprises a polyvinyl alcohol copolymer having enhanced oxidation resistance. The copolymer generally includes sulfonic acid functional comonomer units which provide the copolymer with superior resistance to oxidation. The separator maintains superior oxidation resistance at ambient temperatures and at elevated discharge temperatures. The durability of the inventive separator enables the manufacture of batteries with improved performance as evidenced by increased capacity and cycle life (for rechargeable batteries).